Embodiments of the present invention relate to compositions, systems, and methods, for the pulmonary delivery to a patient, of microstructures in a suspension medium.
The efficacy of many pharmaceutical agents is predicated on their ability to proceed to the selected target sites and remain there in effective concentrations for sufficient periods of time to accomplish the desired therapeutic or diagnostic purpose. Difficulty in achieving efficacy may be exacerbated by the location and environment of the target site as well as by the inherent physical characteristics of the compound administered. For example, drug delivery via routes that are subject to repeated drainage or flushing as part of the body's natural physiological functions offer significant impediments to the effective administration of pharmaceutical agents. In this respect, delivery and retention problems are often encountered when administering compounds through the respiratory or gastrointestinal tracts. Repeated administration of fairly large doses are often required to compensate for the amount of drug washed away and to maintain an effective dosing regimen when employing such routes. Moreover, the molecular properties of the pharmaceutical compound may impair the absorption through a given delivery route, thereby resulting in a substantial reduction in efficacy. For instance, insoluble particulates are known to be subject to phagocytosis and pinocytosis, resulting in the accelerated removal of the compound from the target site. Such reductions in delivery and retention time complicate dosing regimes, waste pharmaceutical resources and generally reduce the overall efficacy of the administered drug.
In this respect, one class of delivery vehicles that has shown great promise when used for the administration of pharmaceutical agents is fluorochemicals. During recent years, fluorochemicals have found wide ranging application in the medical field as therapeutic and diagnostic agents. The use of fluorochemicals to treat medical conditions is based, to a large extent, on the unique physical and chemical properties of these substances. In particular, the relatively low reactivity of fluorochemicals allows them to be combined with a wide variety of compounds without altering the properties of the incorporated agent. This relative inactivity, when coupled with other beneficial characteristics such as an ability to carry substantial amounts of oxygen, radioopaqueness for certain forms of radiation and low surface energies, have made fluorochemicals invaluable for a number of therapeutic and diagnostic applications.
Among these applications is liquid ventilation. For all practical purposes, liquid ventilation became a viable technique when it was discovered that fluorochemicals could be used as the respiratory promoter. Liquid breathing using oxygenated fluorochemicals has been explored for some time. For example, an animal submerged in an oxygenated fluorochemical liquid, may exchange oxygen and carbon dioxide normally when the lungs fill with the fluorochemical. In this regard it has been shown that mammals can derive adequate oxygen for survival when submerged by breathing the oxygenated fluorochemical liquid. In particular, it has been established that total liquid ventilation may keep mammals alive for extended periods prior to returning them to conventional gas breathing.
Those skilled in the art will appreciate that contemporary liquid ventilation is an alternative to standard mechanical ventilation which involves introducing an oxygenatable liquid medium into the pulmonary air passages for the purposes of waste gas exchange and oxygenation. Essentially, there are two separate techniques for performing liquid ventilation, total liquid ventilation and partial liquid ventilation. Total liquid ventilation or “TLV” is the pulmonary introduction of warmed, extracorporeally oxygenated liquid respiratory promoter (typically fluorochemicals) at a volume greater than the functional residual capacity of the subject. The subject is then connected to a liquid breathing system and tidal liquid volumes are delivered at a frequency depending on respiratory requirements while exhaled liquid is purged of CO2 and oxygenated extracorporeally between the breaths. This often involves the use of specialized fluid handling equipment.
Conversely, partial liquid ventilation or “PLV” involves the use of conventional mechanical ventilation in combination with pulmonary administration of a respiratory promoter capable of oxygenation. In PLV a liquid, vaporous or gaseous respiratory promoter (i.e. a fluorochemical) is introduced into the pulmonary air passages at volumes ranging from just enough to interact with or coat a portion of the pulmonary surface all the way up to the functional residual capacity of the subject. Respiratory gas exchange may then be maintained for the duration of the procedure by, for example, continuous positive pressure ventilation using a conventional open-circuit gas ventilator; Alternatively, gas exchange may be maintained through spontaneous respiration. When the procedure is over, the introduced respiratory promoter or fluorochemical may be allowed to evaporate from the lung rather than being physically removed as in TLV. For the purposes of the instant application the term “liquid ventilation” will be used in a generic sense and shall be defined as the introduction of any amount of respiratory promoter or fluorochemical into the lung, including the techniques of partial liquid ventilation, total liquid ventilation and liquid dose installation.
Use of liquid ventilation may provide significant medical benefits that are not available through the use of conventional mechanical ventilators employing a breathable gas. For example, the weight of the respiratory promoter opens alveoli with much lower ventilator pressure than is possible with gas. Additionally, liquid ventilation using fluorochemicals as the respiratory promoter has been shown to be effective in rinsing out congestive materials associated with respiratory distress syndrome. Moreover, liquid ventilation has been shown to be a promising therapy for the treatment of respiratory distress syndromes involving surfactant deficiency or dysfunction. Elevated alveolar surface tension plays a central role in the pathophysiology of the Respiratory Distress Syndrome (RDS) in premature infants and is thought to contribute to the dysfunction in children and adults. Liquid ventilation, particularly using fluorochemicals, is effective in surfactant-deficient disorders because it eliminates the air/fluid interfaces in the lung and thereby greatly reduces pulmonary surface tension. Moreover, liquid ventilation can be accomplished without undue alveolar pressures or impairing cardiac output and provides excellent gas exchange even in premature infants. Finally, fluorochemicals have also been shown to have pulmonary and systemic anti-inflammatory effects.
In addition to liquid ventilation, it has been recognized that fluorochemicals may be effective in the pulmonary delivery of bioactive agents in the form of liquid or solid particulates. For example, pulmonary delivery of bioactive agents using fluorochemical suspensions is described in Sekins et al., U.S. Pat. No. 5,562,608, Fuhrman, U.S. Pat. No. 5,437,272, Faithful et al. U.S. Pat. No. 5,490,498, Trevino et al. U.S. Pat. No. 5,667,809 and Schutt U.S. Pat. No. 5,540,225 each of which is incorporated herein by reference. The bioactive agents may preferably be delivered in conjunction with partial liquid ventilation or lavage. Due to the physical characteristics of compatible respiratory promoters or fluorochemicals, the use of such techniques provides for improved dispersion of the incorporated agent in the lung thereby increasing uptake and increasing efficacy. Further, direct administration of the bioactive agent is particularly effective in the treatment of lung disease as poor vascular circulation of diseased portions of the lung reduces the efficacy of intravenous drug delivery. Besides treating pulmonary disorders, fluorochemical pharmaceutical formulations administered to the lung could also prove useful in the treatment and/or diagnosis of disorders such as RDS, impaired pulmonary circulation, cystic fibrosis and lung cancer. It will also be appreciated that, in addition to the pulmonary route of administration, fluorochemicals could advantageously be used for the administration of compounds via other routes such as topical, oral (e.g. for administration to the gastrointestinal tract), intraperitoneal, or ocular. Unfortunately, regardless of the administration route, the use of fluorochemical suspensions may result in unreliable and irreproducible drug delivery due to the administration of a non-homogeneous dispersion or instability of the particulates in the fluorochemical phase.
More particularly, drug suspensions in liquid fluorochemicals comprise heterogeneous systems which usually require redispersion prior to use. Yet, because of factors such as patient compliance, obtaining a relatively homogeneous distribution of the pharmaceutical compound is not always easy or successful. In addition, prior art formulations comprising micronized particulates may be prone to aggregation of the particles which can result in inadequate delivery of the drug. Crystal growth of the suspensions via Ostwald ripening may also lead to particle size heterogeneity and can significantly reduce the shelf-life of the formulation. Another problem with conventional dispersions is particle coarsening. Coarsening may occur via several mechanisms such as flocculation, fusion, molecular diffusion, and coalescence. Over a relatively short period of time these processes can coarsen the formulation to the point where it is no longer usable. As such, while such systems are certainly a substantial improvement over prior art non-fluorochemical delivery vehicles, the drug suspensions may be improved upon to enable formulations with improved stability that also offer more efficient and accurate dosing at the desired site.